U.S. patent number 3,973,186 [Application Number 05/501,231] was granted by the patent office on 1976-08-03 for gas analyzing method and apparatus for performng the same.
This patent grant is currently assigned to Sagami Chemical Research Center. Invention is credited to Yasuharu Ijuin, Mitsutoshi Tanimoto, Hiromichi Uehara.
United States Patent |
3,973,186 |
Uehara , et al. |
August 3, 1976 |
Gas analyzing method and apparatus for performng the same
Abstract
A gas sample including polar molecules is introduced into a
cavity (an internal space) of a cavity resonator, the cavity being
provided therein with a Stark electrode which is vertically of a
microwave electric field resonating at the microwave frequency, and
a direct current Stark voltage which is applied to the Stark
electrode together with a sine wave modulation voltage is swept,
resulting in a microwave absorption spectrum representing the polar
molecules in the gas sample. The resonance frequency of the cavity
may be made variable to observe various polar molecules
individually.
Inventors: |
Uehara; Hiromichi (Sagamihara,
JA), Tanimoto; Mitsutoshi (Sagamihara, JA),
Ijuin; Yasuharu (Kodaira, JA) |
Assignee: |
Sagami Chemical Research Center
(Tokyo, JA)
|
Family
ID: |
14240854 |
Appl.
No.: |
05/501,231 |
Filed: |
August 28, 1974 |
Foreign Application Priority Data
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Sep 5, 1973 [JA] |
|
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48-99195 |
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Current U.S.
Class: |
324/636; 422/83;
324/300 |
Current CPC
Class: |
G01N
22/005 (20130101) |
Current International
Class: |
G01N
22/00 (20060101); G01R 027/04 () |
Field of
Search: |
;324/58.5R,58.5A,58.5B,58.5C,.5AH,.5A,.5B,.5AC ;23/254E,255E |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Molecular Dipole Moments and Stark Effect" by Townes et al.,
Physical Review, vol. 77, No. 4, Feb., 1950, pp. 500-505, QC-1;
P-4. .
"Plane Parallel Plate Transmission Line Stark Microwave
Spectrograph" by S. A. Marshall, Review of Scientific Instruments,
vol. 28, No. 2 Feb., 1957 pp. 134-137..
|
Primary Examiner: Chatmon, Jr.; Saxfield
Attorney, Agent or Firm: Oblon, Fisher, Spivak, McClelland
& Maier
Claims
What is claimed is:
1. A gas analysing method comprising the steps of
introducing a gas sample into a microwave rectangular cavity
resonator having a Stark electrode;
causing the resonator to resonate at a fixed microwave
frequency;
applying an alternating current Stark voltage as well as a direct
current Stark voltage to the Stark electrode;
varying the magnitude of the direct current Stark voltage; and
observing the absorption spectrum of the gas sample.
2. A gas analysing method as set forth in claim 1 wherein the
resonance frequency of the cavity is varied for analysis of
different molecules.
3. A gas analyser comprising
a microwave rectangular cavity resonator having a Stark electrode
disposed therein,
means for applying an alternating current Stark voltage and a
direct current Stark voltage to the Stark electrode and for varying
the magnitude of the direct current Stark voltage,
means for supplying microwave energy of fixed frequency to the
rectangular cavity,
a phase sensitive detector connected to the Stark voltage applying
means and to the microwave unit, and
a recorder connected to the phase-sensitive detector.
4. A gas analyser as set forth in claim 3 wherein the rectangular
cavity of the cavity resonator comprises an end wall provided with
a coupling hole through which microwave energy is supplied to the
rectangular cavity and a floating contact member disposed to
directly face the end wall with the end wall and the floating
contact member being disposed at opposite ends of the Stark
electrode.
5. A gas analyser as set forth in claim 4 wherein the resonator
includes means for varying the resonance frequency thereof.
Description
BACKGROUND OF THE INVENTION
The present invention relates to an analysis of gaseous polar
molecules, and, in particular, to a simple and high sensitive
measuring method of injurious gas components for human health and
to an apparatus for performing the same method.
It is well known that the polluted city air, factory exhaust smokes
and/or the automobile exhaust gases include injurious substances
for human health and the demand of detecting and measuring these
substances with high sensitivity has been increased recently. For
this purpose, the gas chromatography or the mass spectrometer has
been utilized heretofore. In either of the conventional apparatus,
however, considerably complicated operations are required to
identify these substances contained by a very small amount in the
sample gas having a very complex composition and to measure the
amount thereof.
It has been known that most of the air polluting substances are
polar molecules having dipole moments and have different energy
absorption bands in the microwave region which are inherent to the
respective molecules. One of the important features of the
absorption in the microwave region is that it produces well
resolved spectrum. Therefore, it is easy to identify and measure
some very small amount of substance exactly from the position of
absorption, i.e., absorption spectrum irrespective of the
composition of the sample gas including the substance.
In the conventional gas analyser utilizing the microwave
absorption, a waveguide cell or Fabry-Perot type resonator is used
and the detection of spectrum is performed by sweeping microwave
frequency. In the case of the waveguide cell, the length thereof
must be at least several meters in order to obtain a sufficiently
high detection sensitivity. However, since it is not practical to
produce and to use such long cell, the sensitivity could not be
obtained accordingly. The Fabry-Perot type resonator is essentially
larger in size than the cavity resonator. In the latter case
complicated apparatus and/or complicated regulations of the
apparatus are usually required to provide an appropriate frequency
sweeping.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a gas analysis
method capable of almost completely removing the defects inherent
to the aforesaid conventional methods. Another object of the
present invention is to provide an apparatus for performing the
same method.
These objects of the present invention are achieved by an
employment of a specially designed cavity resonator as a sample
cell. In the previously mentioned conventional techniques, the
absorption spectrum is detected by sweeping the microwave
frequency. In the present invention, the spectrum of an intended
molecule is detected by changing a direct current voltage magnitude
applied to a Stark electrode plate equipped within a cavity
resonator (sample cell) to which a microwave energy of a constant
frequency is applied to make it to resonate at the frequency.
It is well known that the energy level of a certain polar molecule
in an electric field is shifted due to the Stark effect. Therefore,
when a microwave energy is irradiated to the molecule to which the
electric field is applied, the molecule will absorb a microwave
energy of a frequency different from that when no electric field is
applied. Accordingly, when the microwave frequency is fixed at the
resonance frequency of the cavity resonator and when an electric
field is applied thereto to shift the energy level of the molecule
and varied until an interval between the shifted levels becomes
equal to the energy corresponding to the fixed microwave frequency
of the cavity resonator, an absorption will be observed. That is,
instead of the frequency sweeping as in the conventional microwave
gas analyser, the detection, the identification and the
quantitative measurement of gaseous molecules can be performed by
sweeping the Stark voltage.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic block diagram of an embodiment of a gas
analyser constructed in accordance with the present invention;
FIG. 2 is a detailed cross-sectional side view of a cavity
resonator used in the gas analyser in FIG. 1;
FIG. 3 is a detailed, partially cross-sectioned plane view of the
cavity resonator in FIG. 2; and
FIGS. 4 to 9 are examples of experimental results obtained by the
present invention.
DESCRIPTION OF EMBODIMENTS
One example of the cavity resonator is that of TE.sub.lOn mode
having a rectangular cross section, as will be described
hereinafter, and is provided with a sample inlet port for
introducing the sample gas into the interior of the cavity thereof
and a gas exit port. The microwave energy is introduced into the
cavity through a coupling hole. The Stark electrode is a metal
plate and it is disposed along the center line of the shorter side
of the rectangular and vertically of the microwave electric field
within the cavity, with the microwave coupling hole provided at the
center of a cavity end plate. With this arrangement of the coupling
hole and the Stark electrode, the undesirable effect due to the
presence of such Stark electrode in the cavity, which affects the Q
value and the resonance frequency of the cavity resonator, is
eliminated.
FIG. 1 is a block diagram of an analyser constructed in accordance
with the present invention for performing the present method. The
gas analyser shown in this figure comprises a cavity resonator 10
having a rectangular cross section, a microwave source 14, a
detection system 16 including a detector and a lock-in type
amplifier and a recorder 18. Although the cross section of the
cavity is shown as rectangular in this embodiment, it should be
recognized that any other configuration such as circular etc. can
also be used in the present invention without any degradation of
the performance of the analyser. A metal plate is disposed in the
center portion of the cavity resonator 10 in parallel with the
longer side of the rectangular thereof and used as the Stark
electrode. After the cavity resonator 10 is filled with a sample
gas, an alternative current Stark voltage is applied to the Stark
electrode together with a direct current Stark voltage. Since when
the direct current Stark voltage is swept the microwave energy
applied from the microwave source 14 through the coupling hole to
the interior of the cavity 10 is absorbed at a specific value of
the direct current Stark voltage which is inherent to the gas
molecule in question, the absorption is picked-up by the lock-in
amplifier 16 to record it by the recorder 18 as an absorption
spectrum. Thus, the spectrum of the sample gas can be obtained.
FIG. 2 is a cross sectional side view of an embodiment of the
cavity resonator 10 shown in FIG. 1 and FIG. 3 is a plane view of
the same in partially cross section. As mentioned previously, the
resonator is rectangular one operating in TE.sub.lOn mode and has a
cavity region 21 within which the sample gas is introduced. One end
of the cavity 21 is closed by a wall having a coupling hole 24, the
opposite end of which is closed by a floating contact piston member
25. On the right and left inner walls of the cavity 21, dielectric
spacers 27 of such as Teflon (trademark of DuPont) are provided
respectively. Also within the cavity 21, a metal plate 26 is
provided at the center of the shorter side of the rectangular
cavity 21 and in parallel with the longer side thereof and extends
throughout the length of the cavity. The metal plate 26 which is
used as the Stark electrode is vertically of a microwave electric
field produced within the cavity by the microwave supplied from the
microwave source 14 through the coupling hole. The cavity is
further provided with a sample gas inlet 22 and a gas exit 23 as
mentioned previously. The position of the floating contact piston
member 25 within the cavity may be regulatable externally by means
of a suitable regulation means 30. The Stark electrode 26 is
suitably connected through a connector 28 to the Stark power supply
12.
In the present analyser, it is possible to observe simultaneously a
plurality of molecular spectra by sweeping the direct current Stark
electric field within a range from 0 KV/cm to several tens KV/cm
and to identify the molecules from the values of the respective
absorption spectra and the spectral patterns thereof. Particularly,
in a case where a specific molecule is to be observed, the
resonance frequency of the resonator may be set at or around an
absorption frequency of the molecule under no electric field. By
this setting, such high Stark field voltage as several tens KV/cm
will become unnecessary. Therefore, it is preferable to make the
resonance frequency of the cavity resonator of the present
invention analyser tunable within a certain range. This is easily
achieved by providing the floating contact piston member 25 which
is shiftable in the longitudinal direction of the cavity at one end
of the cavity opposite to the end thereof having the coupling hole
24.
FIGS. 4, 5 and 6 show spectra obtained by sweeping the D.C. Stark
voltage for various resonance frequencies.
FIG. 4 is the Stark sweeping spectrum of acrolein obtained by the
present apparatus shown in FIG. 1 under the conditions of fixed
8902.50 MH.sub.Z resonance frequency and 0.11 Torr cavity inner
pressure.
FIG. 5 is the spectrum of methylisocyanate where the resonance
frequency is fixed at 8740.5 MH.sub.Z.
FIG. 6 is that of ammonia where the resonance frequency is 8826
MH.sub.Z and the inner pressure is 0.102 Torr.
FIGS. 7 and 8 show further examples of the Stark sweeping spectra
which demonstrate the remarkably high sensitivity of this
spectrometer. FIG. 7 is that of formaldehyde in a standard air
sample including 72 ppm formaldehyde with the resonance frequency
being 8886.9 MH.sub.Z and FIG. 8 is that of formaldehyde directly
obtained by using an automobile exhaust gas as a sample gas, the
magnitude of this spectrum corresponding to 24 ppm. The
signal-noise ratio in FIGS. 7 and 8 has led to the minimum
detectable absorption coefficient of 6.0 .times. 10.sup..sup.-13
cm.sup..sup.-1.
FIG. 9 is another spectrum of formaldehyde of 0.5 ppm.
As described hereinbefore, since the present invention is a
spectrometer using microwave energy, the identification of
molecules is precise. In addition to this advantage, since the
present analyser utilizes the cavity resonator, it is very compact
and the sensitivity thereof is very high. Furthermore, since the
sweeping is to simply vary the D.C. Stark voltage, the construction
of the analyser itself is very simple. Other advantages than those
mentioned as above will be obviously appreciated by those skilled
in the art.
* * * * *